Hydraulic systems are known for converting fluid power, for example, pressurized flow, into mechanical power. Fluid power may be transferred from one or more hydraulic pumps through fluid conduits to one or more hydraulic actuators. Hydraulic actuators may include hydraulic motors that convert fluid power into shaft rotational power, hydraulic cylinders that convert fluid power into translational power, or other hydraulic actuators known in the art.
In an open-center hydraulic system, fluid discharged from an actuator is directed to a low-pressure reservoir, from which the pump draws fluid. Controlling an operation of a hydraulic actuator in a hydraulic circuit is conventionally accomplished using a single spool-type valve. The single spool valve has a series of metering slots which control flows of hydraulic fluid in the hydraulic circuit, including a flow from a pump to the hydraulic actuator and a flow from the hydraulic actuator to a tank. When the hydraulic actuator is a hydraulic cylinder, these flows are commonly referred to as pump-to-cylinder flow and cylinder-to-tank flow, respectively.
The metering slots may be machined into the stem of the spool valve. With this arrangement, slot timing and modulation are fixed. Thus, in order to modify the performance of a hydraulic circuit including such a spool valve, the stem may require additional machining. Furthermore, in order to add additional features to the performance of the hydraulic circuit, an entirely new stem may be required. In turn, adding features to or optimizing the performance of conventional hydraulic circuits may be expensive and time consuming.
Hydraulic systems with independent metering valves (IMVs) provide an operator with the ability to modify the performance of the hydraulic circuit without modifying hardware. In a hydraulic system with IMVs, each IMV includes four independently operable, electronically controlled metering valves to control flows within the hydraulic circuit. Two of the metering valves are disposed between the input port and the control ports. The other two metering valves are disposed between the output port and the control ports. Because each of the metering valves is controlled electronically, the performance of the hydraulic circuit can be modified by adjusting a control signal to one or more of the metering valves. IMVs operate in a closed-center system that is incompatible with conventional open-center systems. An example of this incompatibility is that priority in an open-center system is determined by proximity to the flow source of the hydraulic fluid while in closed-center systems, priority is a function of the programming in the control module.
U.S. Pat. No. 6,880,332 (hereinafter “the '332 patent”), titled “Method of Selecting a Hydraulic Metering Mode for a Function of a Velocity Based Control System,” purports to describe a hydraulic system with an IMV in which the metering modes can be varied according to the task. However, while the '332 patent offers certain advantages over conventional spool-type valves, the hydraulic system of the '332 patent includes particular electrohydraulic (EH) control systems that are not present in conventional hydraulic systems. As a result, the hydraulic system of the '332 patent may be incompatible with conventional hydraulic systems incorporating spool-type valves.
Accordingly, there is a need for improved hydraulic systems to address the problems described above and/or problems posed by other conventional approaches.